Talking about Flexo Printing Platemaking Technology

Flexo printing, offset printing, gravure printing and screen printing are the four most popular printing methods in the world. They have their own characteristics. Because flexographic printing has unique flexibility and economy, and it is beneficial to the protection of the environment, it meets the hygienic standards for food packaging prints, which is one of the reasons for the rapid development of flexographic printing in foreign countries. However, from the current situation our country, comparing the popularity of offset printing, gravure has occupied a large market in the packaging industry, and flexographic printing technique is relatively late start, although in recent years have indeed made Great progress, but there is still a big gap with the international advanced technology.

First, the flexo plate making process:
Design and plate-making process from a document perspective, flexographic printing process self-contained, has its own unique, substantially plate-making process as follows:

Document è è electronic color photographic positive or a negative back exposure è è è main exposure development processing after drying è è è after treatment compared with exposure è pasting offset plate, there is a difference mainly in the following aspects:

1. Reproducible range of color values. Offset of 1% to 99% (or 2% to 98%), flexo 3% to 95%.
2. Expanded outlets (50%). Offset 15% to 20%, flexo 30% to 40%.
3, the number of screen lines. Offsets are generally up to 175 lines per inch, while flexo screens generally do not exceed 150 lines per inch.

Second, the size of the color separation sheet deformation The most obvious feature of the flexographic plate is elastic, when the flexographic plate is mounted on a cylindrical roller, the plate produces a curved deformation along the surface of the cylinder, and this deformation affects the pattern on the surface of the printing plate. With text, the printed artwork is not a correct reproduction of the design manuscript, even a serious distortion. Such static deformation (tensile deformation) generated in the circumferential direction of the drum after the flexographic plate is mounted on the drum is always unavoidable. In order to compensate for the distortion of the printed image, the size of the corresponding graphic text on the negative plate must be reduced. Before designing a manuscript or color separation, the amount of elongation of the plate should be taken into account. The corresponding value should be subtracted from the circumferential length of the manuscript for compensation so that the printed product meets the dimensional requirements.

In addition to the reduction ratio, it is related to the radius of the platen roller and the thickness of the double-sided tape, and also related to the thickness of the plate. When the plate is exposed to a flexographic plate, the following formula is generally used to calculate the reduction ratio of the color separation plate:
Reducing ratio (percentage) = K/R*100%

Among them, R is the printing length of the plate cylinder, K is the coefficient, it depends on the thickness of the plate used, for example, when the thickness of the plate is 1.70mm, the K value is 9.89mm; and when the thickness of the plate is At 2.29 mm, the K value is 13.56 mm.

Third, the flexible version of the transmission network

1. Reasons for the expansion of outlets The expansion of printing outlets is inevitable. There are two main reasons for the expansion of outlets: one is for physical reasons, or it is a mechanical reason. At the moment of imprinting, the printing outlets The ink will have a certain deformation due to the effect of the extrusion, which will cause the dot to enlarge; another reason is the optical reason, that is to say the dot enlargement is caused by the reflection of the light, and the light is at the edge of the ink dot of the dot. Scatter occurs, resulting in a considerable dot gain in vision. Optical dot enlargement depends on the transparency of the ink, the smoothness of the paper, absorption properties, and the like.

2. Flexonet transmission rules:
In flexo printing, because the elasticity of the photosensitive resin plate used is relatively large, and a certain printing pressure needs to be applied during the printing process, even though light pressure printing is used in the flexographic printing, it still leads to image dot on the printed matter. The extended image expands and causes changes in color and gradation replication. In practice, by measuring and plotting the corresponding flexographic characteristics, it can be seen that the dot expansion in the flexographic printing process is very serious, and the dots below 10% are difficult to control. Therefore, special treatment should be performed for the high gloss region. Should try to flatten the net.
Compensation for dot gain can be done at the scanner side or on the imagesetter, preferably on an imagesetter. The edges of the small dots generated after exposure are relatively neat and of good quality.

3. Influencing factors (1) The number of screen lines increases. The higher the number of screen lines, the more serious the network expansion.
(2) The size of the printing pressure, the greater the printing pressure, the more serious the dot expansion, and vice versa, the smaller the dot expansion. Therefore, you should try to maintain "zero pressure" in flexographic printing.
(3) Dot shape. Circular dots, square dots, chain dots, and elliptical dots have different dot gains in different gradations. In the flexographic printing, the chain-shaped dots are commonly used; for the small dots in the high-light area, the use of FM dots (FM) works best.

Fourth, the production of flexible version At present, the flexographic printing used in the plates are basically solid photosensitive resin plate, the photosensitive mechanism is: the photosensitive resin under a certain amount of light irradiation, the rapid decomposition of the molecules, resulting in lively and extremely unstable The high-energy radicals (radicals) and high-energy radicals re-initiate polycondensation of the unsaturated bond-containing resin. The flexographic platemaking process mainly includes the following processes:

1. The backside exposure of the plate is to determine the height of the relief on the plate, ie the depth of corrosion, and to cure the substrate.

2. Put the printing plate and the negative film together, use the ultraviolet light for front exposure, form the graphic part on the printing plate, and make it solidify.

3, the printing plate is placed in a solvent brush, the purpose is to brush the unexposed part of the plate material, so that the graphic part of the relief.

4. The printing plate is dried in a dryer to promote the volatilization of the solvent absorbed in the printing plate as soon as possible so that the thickness of the printing plate can be restored to its original standard value.

5, after the exposure and detackification processing. The post-exposure and de-tack treatment of the dried plates can further solidify the shoulders and bases, improve the flexographic printing performance, and increase the flex print resistance.

V. Points to note in the flexo platemaking process:
1. Try not to place fine details such as small prints, dots, etc. on a large area on the ground. Even the same color should be divided into two editions. If it is impossible to separate them (for example, the number of printer color groups, etc.), consider Appropriately make padding locally.

2, try to avoid large areas of real-color multicolor overprint.

3, the text specifications can not be too small, even more so with the negative text, otherwise, when the print pressure changes, the printed text shows a larger amount of deformation, so that Yang Wenwen thicker, Yin Xie text thin or paste dead.

4, the width of the independent fine lines should be greater than 0.2 mm.

5. In the flexographic printing, the engraving angle of the ink hole on the anilox roller is generally 45°. Therefore, when the ordinary anilox roller is used for printing, the printing plate should avoid the 45° angle of the cable. Avoid print moire.

6. Avoid designing wide and long bars and solids in the horizontal direction of the print cylinder. This can cause machine vibration. Ideally, slashes, curves, wave lines, and other irregular curves.

7, the layout to avoid designing a larger circular pattern. Because when the plate is elastically stretched or elastically compressed, the regular geometric pattern becomes irregular and the circle becomes elliptical.

8, to avoid the strict requirements of strict seams.

9, the original color to take into account that the press can print a few colors.

9, in the use of ink overlay color, should not use two equal color blocks overlap printing, in order to avoid overprinting is not allowed to affect the printing quality. It is possible to use overly large areas of solid patches to overprint text or patterns and to overprint local patches.

Six, flexible plate making common faults and solutions
1, the layout sticky

the reason:
1) The plate is not cleaned after rinsing;
2) The rinse solution has been used for too long or has insufficient active ingredients.

Solution:
1) Plate flushing corrosion, and then thoroughly clean;
2) Replace flushing solvent.
2, the straight line is not straight, curved or wavy

the reason:
1) After exposure is not sufficient;
2) Insufficient main exposure;
3) Insufficient drying time.

Solution:
1) Adjust the exposure time;
2) The main exposure time is tested again;
3) Increase the drying time.
3) Missing fine text and thin lines

the reason:
1) After exposure is not sufficient;
2) The main exposure is insufficient.

Solution:
1) Re-adjust the post-exposure time;
2) Perform the main exposure test again.